1. Field of the Invention
This invention relates generally to radio/microwave frequency hardware. In particular, it relates to a dielectric that prevents moisture from entering coaxial cables and methods of manufacturing cables incorporating such a dielectric.
2. Description of Related Art
Coaxial cables are widely used for the transmission of analog and digital signals at radio and microwave frequencies. A typical coaxial cable consists of a metallic inner conductor, a dielectric material, and a metallic outer conductor arranged in a circular, concentric manner. The signal transmitted across the cable appears as an electromagnetic field in the dielectric, causing electrical currents to flow through the inner and outer conductors. During transmission, the signal may experience attenuation due to the resistance of the inner and outer conductors and the loss factor of the dielectric material.
In order to minimize the transmission loss of signal, artisans may select particular materials for coaxial cables. The materials for the inner and outer conductors are chosen to minimize resistance. A designer may also pick materials having the lowest dielectric loss. The dielectric material should also be selected for minimal permittivity.
Permittivity values describe how well an electric field can permeate a dielectric material. A perfect dielectric would have no conductivity, so it would be able to store and return electrical energy as an ideal capacitor. Real dielectrics have some conductivity, so the electrical current will not be entirely confined to the inner and outer conductors of the coaxial cable.
For the inner and outer conductors, economic and mechanical constraints usually result in the selection of a particular type of metal. Silver has the highest electrical conductivity of any metal. Copper, gold, and aluminum also have high conductivity values.
A dielectric constant, also known as “relative permittivity,” is used to measure the relative effectiveness of a dielectric. By definition, an absolute vacuum has a dielectric constant of 1. Air, having a dielectric constant of 1.0054, has similar electrical characteristics to a vacuum. However, something other than air must be placed between the inner and outer conductors to ensure their mechanical stability. In particular, the dielectric layer should ensure that the conductors remain concentrically aligned.
Coaxial cables that use air as a dielectric have very good signal propagation characteristics. However, such cables are quite vulnerable to bending, as air is unlikely to stop the inner and outer conductors' from contacting each other if the cable is abruptly bent. In addition, the electrical performance of an air-filled cable will deteriorate rapidly if any moisture intrudes.
In contrast, coaxial cables using a foam dielectric type possess significantly better bending properties than air dielectric cables. Cables which use a solid polymer dielectric are also less expensive, but are less efficient at transmitting and receiving signal because air has a much lower dielectric constant than solid polymers. Therefore, most designers prefer using a foam dielectric instead of a solid polymer.
Other coaxial cables may contain polyethylene or another resin in their dielectric layers. Such cables often require application of antioxidants to provide protection against oxidative degradation of their resins. These cables may also be vulnerable to moisture migration between the insulation and the inner and outer conductors. Moisture may react with the metallic surface of the conductors, causing corrosion to develop.
High frequency coaxial cables may use dielectric materials such as polyethylene (PE) and polytetrafluoroethylene (PTFE), and substances derived from PE or PTFE. These materials have relative permittivity values in the 2.0 to 2.4 range. The relative permittivity of these substances can be further reduced by adding air.
For example, the plastic might be extruded to convert it into foam. Alternatively, microscopic fissures could be created in the material to admit air. These techniques can only add a limited amount of air without impairing the dielectric's ability to provide mechanical stability. In particular, if too much air is added, the inner and outer conductors will not remain in place if the coaxial cable is bent or twisted.
Coaxial cables that use air as a dielectrics need to prevent moisture from entering the air pockets. If water collects in these spaces; it may significantly degrade the quality of the cable. More specifically, water can significantly increase the dielectric constant, thereby producing power loss and corrosion of the metallic conductors. Accordingly, there is a need for a coaxial cable with low loss that prevents the intrusion of moisture into the dielectric.
Water vapor is known to enter coaxial cables in several ways. It can diffuse through the jacket surrounding the outer conductor or through holes that form in the jacket. Even worse, water can flow into the cable if a terminal end is not sealed. In such cases, water can quickly fill the gap between the inner and outer conductors, causing the dielectric constant to rise rapidly. Thus, there is a need to limit water intrusion into the dielectric layers of a coaxial cable.